pc: Factor common code out of pc_boot_set() and cmos_init()

Code duplicated in commit 0ecdffbb.  The two versions are similar, but
not identical:

* cmos_init() reports errors to stderr, pc_boot_set() via
  qemu_error().  The latter is fine for both, so pick that for the
  common code.

* cmos_init() obeys fd_bootchk, pc_boot_set() ignores it.  Make it a
  parameter of the common code.